1. Field of the Invention.
This invention relates to techniques for detecting defects in the thin layer of magnetic material which serve as the magnetic storage media for a rigid disk drive, system.
2. Prior Art.
Data information is stored in a magnetic media by magnetizing a small area of the magnetic media. Small, random defects or flaws in the thin layer of magnetic material for a rigid disk can result in storage and subsequent playback of erroneous data bits. These erroneous data bits are created when data information is initially written into a defective area of the disk. These erroneous data bits are then subsequently read out from the disk. A data bit error for a particular bit corresponding to a particular area of the disk is caused either by the magnetization for a bit being missing or by magnetization being added at the storage location. In order to test and identify defects in the thin layer of magnetic material of a disk, a typical prior art technique is to perform a surface analysis of the thin layer of magnetic material on the disk and to produce an error map for the recording surface of the disk. The error map is then used to avoid the defective areas of the disk during subsequent recording and playback of data information. To perform a surface analysis, the disk is formatted and the locations of defects are stored in the header fields at the beginning of the data records stored on the various tracks of a disk. During read/write operations, the headers are looked at to avoid the defective recording areas on the disk.
To detect flaws in a :magnetic media for a disk storage device, a test signal such as a high frequency, alternating data pattern is written onto the disk. This pattern is then read out of the disk as a high frequency output test signal which has a sinusoidal waveform. This high frequency output test signal with its sinusoidal waveform is then observed for deviations from an expected sinusoidal waveform to indicate the occurrence of a defect on the disk. Previously, it was necessary to write and rewrite such a test pattern on the disk a number of times. This was because the previous flaw-detection systems used narrow-band tracking notch filters to remove the expected sinusoidal signal and to pass only those sidebands representing error information. A narrow-band tracking filter, which uses a delay line, is disclosed in U.S. Pat. No. 4,929,894 entitled "Method and Apparatus for Increasing Throughput on Disk Drive Quality Control Testing" by M. Monett. In the frequency domain, the defect, or error, information is in the form of sidebands around the carrier. These sidebands are produced by amplitude and phase modulation of the test pattern by the defects on the disk. Previous detection systems were only amplitude-sensitive and not phase-sensitive. This meant that defects causing phase errors went undetected. Consequently, it was necessary to write and rewrite the test signal a number of times in order to find the defects.
U.S. Pat. No. 4,881,136 entitled "Method and Apparatus for Detecting Minute Defects on Magnetic Disk by Monitoring Both Amplitude Decrease and Phase Shift of a Reproduced Signal" to Shiraishi discloses a disk-error detection system which makes repeated passes to detect errors in a continuous signal and which uses peak-to-peak amplitude detectors.. An analog signal-processing apparatus for detecting a flaw in a magnetic media is disclosed in U.S. Pat. No. 5,121,057 titled "Media Flaw Detection Apparatus for a Magnetic Disk Drive with Squaring and Summary of In-Phase and Quadrature-Phase Detected Signals" to Huber et al. The Huber 057 patent uses analog techniques for detection of flaws in a magnetic media. In one embodiment disclosed in the Huber 057 patent, an analog input signal is multiplied by an in-phase reference signal and also by an analog quadrature-phase reference signal, using a pair of balanced modulators as analog multipliers. The analog out,put signals from each of the analog multipliers are then squared and summed together to produce an analog signal which is representative of a flaw in the magnetic media. In another analog embodiment disclosed in the Huber 057 patent, an analog input signal is squared using a balanced modulator. The analog input signal is also phase shifted by 90 degrees and the phase-shifted signal is also squared. Both of the squared analog signals are then summed together. The summed analog signal is then low-pass filtered to provide an analog signal which is representative of a flaw in the magnetic media.
A need exists for digital techniques to detect flaws in a magnetic storage media, where the digital technique uses digitized, discrete time signals and digital signal processing techniques. A need also exists for a simplified technique for processing a playback signal from a magnetic media which contains a flaw. A need also exists for a phase-insensitive system for detecting flaws in a magnetic media, which system does not require phase synchronous sampling of the input signal.